Heat sealed pouches made from flexible sheet stock are now being used for packaging certain products and sterilizing them in the package. The packaged product may then be stored without refrigeration. Typical products packaged and used in this manner are certain foods and medical supplies. The packaging thus used is known generally as a retort pouch.
The first generation of retort pouches included certain reactive adhesive compositions, used for adhering the various layers to each other. In subsequent developments, the reactive adhesives were eliminated because of possible extractive contaminants, and retort pouch structures are now made by various methods of adhering the layers to each other by one or more of several extrusion and/or lamination processes. Due to recently developed adhesive compositions, adhesive lamination is again a viable laminating process. Illustrative of the extrusive-type processes is one taught in U.S. Pat. No. 4,190,477. In the process described in that patent, biaxially oriented polyester is first adhesively mounted to metal foil on what is to be the outside of the package. A polypropylene-based heat sealing layer is separately extruded. A primer is applied to the side of the foil opposite the polyester. Finally the sealant layer and the primed foil are extrusion laminated together using an extrusion grade polypropylene-based polymer as the extrusion laminant.
Another retortable structure, and not including metal foil, is disclosed in U.S. Pat. No. 4,405,667. In that teaching the outer layers of the contemplated pouch include a /nylon/EVOH/nylon/ sub-structure, where EVOH is ethylene vinyl alcohol copolymer.
Non-foil retortable sheet structures have a number of potentially attractive advantages. Foil is the only component of known foil-containing structures that prevents microwave energy from penetrating the package. Thus, elimination of the foil layer gives the structure additional functional utility as for cooking with microwave appliances. There is also the functional potential for transparency of the pouch, as for visual inspection of the contents.
Non-foil structures have substantial potential for cost advantages. The foil material, itself, is costly. Also, the processes for making non-foil structures are potentially less expensive. As multiple layer coextrusion technology continues to develop, the cost of making increasingly complex polymeric structures is continuing to decline. As a result, a significant advantage for the non-foil structures is that the number of processing steps is usually less for construction of a non-foil pouch structure than for construction of a foil-based structure having generally comparable functional characteristics. Thus the potential for cost-competitiveness generally favors non-foil structures.
Common to the requirements of retort pouch packaging is the requirement that the filled and sealed package be subjected to sterilizing conditions of relatively high temperature after the pouch is filled with product and sealed. Typical sterilizing conditions are on the order of 250.degree. F. to 260.degree. F. and may range in severity up to about 275.degree. F. with residence times at sterilizing conditions of as much as 30 minutes or more. Such conditions impose severe stresses on the packages. Many packaging structures provide excellent protection for the package contents at less severe conditions. For example, relatively simple packaging structures for packaging requiring the ability to withstand boiling water, such as at 212.degree. F. are readily available from several suppliers. However, such relatively simple packaging structures are not capable of surviving the higher temperatures of retort processing, and the subsequent handling while they are still hot. For satisfactory performance in a retortable package, the structure must not delaminate, shrink, or wrinkle, or otherwise be adversely affected by the retort processing. Oxygen and water barrier properties are typically required of retortable packaging structures. These barrier properties must not be permanently adversely affected by the retort processing and attendant handling of the packages. And finally the structure must be adequately strong to permit handling of the package while still hot.
Each of the several layers of the sheet or film structure serves particular purposes and must satisfy particular requirements. The following description of requirements and preferred materials begins with the interior barrier layer.
The barrier layer provides a sufficient barrier to oxygen to provide adequate shelf-life, dependent on the oxygen sensitivity of the product in the package. EVOH provides superior oxygen impermeability when compared with other polymeric materials such as saran and acrylonitrile which have been employed in packages for oxygen barrier qualities. A very thin layer of EVOH will provide an adequate barrier to oxygen. And while EVOH is very expensive compared to an equal quantity of saran or acrylonitrile, the quantity of EVOH required is substantially less. Thus the cost of an EVOH barrier layer is competitive with saran and acrylonitrile because less EVOH is used in a barrier layer than is used in an equivalent layer of saran or acrylonitrile.
The oxygen barrier quality of EVOH is adversely affected by the presence of water in the EVOH layer. A small quantity of water will raise the moisture content of a thin layer of EVOH to an extent where the oxygen barrier quality of the layer is severely affected. The oxygen barrier quality of EVOH is restored when the moisture is removed.
Polyolefins such as polyethylene, polypropylene, and blends and copolymers of the two are considered to be excellent moisture barriers. Polyolefins are heat sealable. High density polyethylene, polypropylene, and blends and copolymers of polypropylene and polyethylene melt at temperatures sufficiently high to provide heat seals which survive hot filling at temperatures in the neighborhood of 190.degree. F. or retorting at up to about 275.degree. F., yet seal at temperatures between 350.degree. F. and 400.degree. F. which are attainable with existing heat seal equipment at normal sealing pressures and time. Consequently, high melting temperature polyolefins are suitable for use as the inside heat seal layer of a retortable pouch. However, the permeability of polyolefins to water is greatly increased at the elevated temperatures encountered in commercial retort-type sterilization. Thus, a polyolefin layer may permit intrusion of water into the EVOH oxygen barrier layer during hot filling or retorting. Since polyolefins again become relatively impermeable to water upon cooling, moisture in the EVOH layer cannot escape through the polyolefin layer. Thus, the layer or layers which are on the outside of the package from the EVOH layer should desirably be relatively permeable to moisture under normal storage conditions to permit escape of any moisture which may be in the EVOH layer.
A suitable polymeric material for the package outside layer is nylon. Nylon is sufficiently permeable to moisture to permit the escape of moisture from the EVOH layer to restore the oxygen barrier quality, yet is sufficiently waterproof to permit accidental wetting of the package without harm to the oxygen barrier property of the EVOH layer. Nylon is tough, flexible, not greatly affected by heat or cold. It is abrasion resistant, transparent, and can be printed upon for label purposes. Nylon is not excessively expensive and has adequate adhesion to EVOH. Nylon can be coextruded with EVOH and with polyolefins. Thus, nylon has several attributes which are desirable in a multiple layer film where the nylon is an outside surface layer, overlying an EVOH barrier layer.
Some structures containing nylon, however, have exhibited a particular problem with embrittlement of the nylon and the EVOH during the retort processing. While these structures may have good utility before retort processing, they do not have satisfactory toughness and abuse resistance after having been subjected to retort processing. While the magnitude of the problem may be somewhat reduced by careful compounding of polymer and additives, it would be desirable to resolve the problem while retaining as much freedom as possible in the compounding of the structural layers.
It is an object of this invention to provide a multiple layer sheet structure containing adjacent nylon and EVOH layers wherein the nylon and EVOH may be characterized by their toughness after retort processing.
It is another object of this invention to provide a multiple layer sheet structure containing adjacent nylon and EVOH layers wherein the nylon and EVOH layers may be characterized by their resistance to impact after exposure to retort processing conditions.
It is a further object to provide a multiple layer non-foil sheet structure suitable for making retortable pouches, the sheet structure containing nylon and EVOH layers and wherein the nylon and EVOH retain their toughness and flexibility, without stress cracking, after exposure to retort processing conditions.
Further objects of the invention are found in methods of making retortable sheet structures containing nylon and EVOH, and wherein the nylon and EVOH substantially retain their respective properties of toughness and flex crack resistance.